It is known by the Article published in "Journal de l'Equipement Electrique et Electronique" No. 306, November 1970, pages 116-117, equipments which are temperature compensated for sensing movements of moving surfaces like axles.
It is disclosed a method of compensating a thermal derivation by using two transducers located in two branches of an electronic bridge, see FIG. 1, and sufficiently close so that they are supposed to be at the same temperature.
One of the transducers named as reference, see FIG. 2, is located in front of a gap which is known and fixed.
FIG. 3 shows a total thermal compensating bridge with inductive coils.
In that document, the two transducers are fixed and are separated by a known fixed distance.
It is also known by German Patent Application DE-A-2550427, corresponding to U.S. Pat. No. 4,160,204, a non-contact distance measuring system comprising an electronic bridge particularly seen in FIGS. 1 and 3, having branches wherein branch 20 is a distance related impedance and the other branches 14, 16, 18 have a fixed impedance, branch 18 serving as a reference impedance for comparing signal changes across branch 20 caused by the impedance change of transducer 22, see column 2, lines 29-48.
The present invention differs from that document by the use of a flexible film of a thin metallic foil which is essentially movable and deformable in space and is a part of a support element for supporting an element to be supported in relation with the volume and shape thereof, in particular a patient P in relation with his or her morphology, and it was unobvious for one skilled in the art that the principle of the non-contact distance measuring system as disclosed in that document could be used in such a technical field, in particular a medical field and applied to the measurement of the positioning of an element acting on an essentially flexible and deformable in space supporting device, i.e. wherever controlled deformability is a requirement.
Other distance sensors using a bridge circuit incorporating variable capacities are also disclosed in U.S. Pat. No. 5,412,327 and its counterpart DE-4,231,816.
Document EP-A-0 218 301 discloses a method and a device for improving support of a patient, which support comprises a mattress provided with one or more closed chambers filled with a fluid under a controlled filling pressure, and uses a measurement device based on a combination of two induction coils 24, 25 shown in particular in FIG. 4 of that document. That device is not very satisfactory because it requires provision to be made to power both induction coils, i.e. including the moving induction coil which is disposed close to the patient, and that always poses a safety problem.
In Document FR-A-2 718 347=EP-A-676 158, the applicant discloses a method and apparatus for supporting an element to be supported, in particular the body of a patient, making it possible to support the element at an essentially constant controlled penetration depth by means of a measurement device comprising an induction coil combined with a piece of metal foil situated under the body being supported, the metal foil being displaced by penetration of the body being supported. Displacement modifies the self-induction coefficient of the induction coil, shifting the resonant frequency of the LC circuit away from the tuning frequency of the oscillator, thereby damping the signal delivered to an amplifier by the oscillator.
The applicant's prior device therefore suffers from the drawback that operation thereof is based on a non-desirable frequency variation. Furthermore, the voltage value obtained is low. In addition, the frequency variation is difficult to make compatible with electromagnetic safety standards. Finally, the frequency variation measurement is sensitive to surrounding metal masses.